While we've been hacking and spluttering, something interesting has happened in the great newspaper paywall debate: the first quarter of subscription figures from the New York Times, and perhaps the first clear indication that there may be life behind a wall. Digital subs to nytimes.com up from nothing to 224,000 customers in three months; 57,000 iPad and Kindle users joined; 750,000 print subscribers registering for online access; plus an extra 100,000 readers taking advantage of special Lincoln car ad sponsorship.

In sum, well over a million newcomers and old faithfuls have done the digital thing. There's been a seeming surge of interest in the screen version of a general-interest newspaper. And the most significant indicator of the lot – a "faint scent of success" as the Guardian puts it – is what's happened to online advertising in the NYT through these initial months of test and anxiety. At group level, it's gone up by 16%. It has not fallen back as fewer unique users have clicked on and, after a little heavy research, banged their heads against a paywall. On the contrary, advertising has taken the route least expected. Money from subscriptions: up. Money from ads: up. Oh! and all that appears to mean that total circulation revenue has edged up a little as well.

Now, treat every fact cited here with profound caution. It could be a blip, the product of a marketing burst, the backwash from all that Lincoln promotion. But it could also hint at some perfect outcome: unique users not too far down, because the New York Times's formula – 20 free articles a month before your credit card chips in – allows them plenty of casual leeway, but enough subscriptions in every form (at a maximum of $35 a month for the total package – laptops, smartphones, tablets) to make promising critical mass.

Compare and contrast the London Times's experience, lurking beyond a far more formidable pay barricade. The last-released figures there show 100,000 signed up, but with rate of growth slowing as the initial year of trial went by. And, at £2 a month, the deal is too cheap to make much difference. Of course you don't sniff at a possible £10.5m a year in extra revenue once all the numbers and special offers settle down. But it's not going to revolutionise the fortunes of a daily and Sunday losing over £40m between them at the last count – especially when those titles have been shedding print sales too fast for comfort.

The UK message of the Times, thus far, is cautious, going on only modestly cheerful, particularly since advertising figures aren't supplied. The US message of the New York Times, though, has been buoyant enough to spark much enthusiasm. Perhaps these walls won't come tumbling down. Perhaps the latest figures from the FT – 34% more digital subscriptions, now touching 230,000, and free registered users up to 3.7 million in the last six months – may bring added confidence outside the specialist financial sector. Perhaps there is a future after all.

But, simply because of so many mopped brows, beware facile optimism. Even the NYT figures need a great deal of context. How much of a potential gain do they really represent? Factor in declining print sales and (on Columbia Journalism Review estimates) the upside is worth somewhere between $34m and $55m a year. But $55m a year is only 3.5% of the paper's $1.55bn in total revenues for 2010. And the Grey Lady lost $114m in the same second quarter that showed the digital subscription surge.

There's a basic equation here. Say a UK national newspaper needs a notional £250m in revenue a year to break even, and that 50% of this sum (during an economic downturn) comes via cover price plus subscriptions, with the rest from ads. Perhaps a recovering economy might push the ad take back up towards its old 80:20 ratio, but that looks a long way off.

So you need £125m each from ads (plus marketing offers) and the same from circulation. Sales money sinks as that circulation declines, and no current experiment with paywalls produces anything like enough to compensate and then replace it. The reliance on print for cover price cash is almost absolute.

Can ads take up the whole of the strain, then? That's a huge question, especially as digital advertising commands somewhere between 10% and 20% of the price of the same ads in print. The FT and Wall Street Journal may net tidy digital sums, but they need newsprint revenues, too. The Mail, the biggest free online newspaper site in Britain, may threaten to pass 80 million unique visitors a month but can still only boast ad revenues of £18m a year. And the cost of designing, developing and marketing new sites for new applications is a constant drain.

So watch euphoria drain away. Interim conclusion: neither digital ad cash nor digital subscriptions via a paywall are in anything like the shape that will be needed for them to take the strain if a print presence is wiped away. The New York Times experience shows a useful willingness to pay something. The soaraway numbers of Mail Online show that free access to everything doesn't automatically bring big advertising dividends. Progress, when you can find it – and UK papers online mostly lost user numbers month-on-month in June – is laborious and still not making big money. It's an infernally difficult balancing act to get right.

You may travel a tad more hopefully up Madison Avenue, perhaps: but the journey is long and the road deeply rutted.